In the manufacture of optical fiber, a glass preform rod which generally is manufactured in a separate process is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and optical fiber is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower.
Because the surface of the optical fiber is very susceptible to damage caused by abrasion, it becomes necessary to coat the optical fiber, after it is drawn, before it comes into contact with any surface. Inasmuch as the application of the coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must become solidified rapidly before the optical fiber reaches the capstan. This may be accomplished by photocuring, for example.
Those optical fiber performance properties which are affected most by the coating are strength and transmission loss. Coating defects which may expose the optical fiber to subsequent damage arise primarily from improper application of the coating material. Defects such as large bubbles or voids, non-concentric coatings with unacceptably thin regions, or intermittent coatings must be overcome. The problem of bubbles in the coating material has been recognized for some time. One solution is to use a pressurized coating material which is fed upwardly ina coating applicator to strip bubbles from the optical fiber. Intermittent coating is overcome by insuring that the fiber is suitably cool at its point of entry into the coating applicator to avoid coating flow instabilities. Coating concentricity can be monitored and adjustments made to maintain an acceptable value. When it is realized that the coating thickness may be as much as the radius of an optical fiber, it becomes apparent that non-concentricity can cause losses in splicing, for example.
Optical fibers are susceptible to a transmission loss mechanism known as microbending. Because the fibers are thin and flexible, they are readily bent when subjected to mechanical stresses, such as those encountered during placement in a cable or when the cabled fiber is exposed to varying temperature environments or mechanical handling. If the stresses placed on the fiber result in a random bending distortion of the fiber axis with periodic components in the millimeter range, light rays, or modes, propagating in the fiber may escape from the core. These losses, termed microbending losses, may be very large, often many times the intrinsic loss of the fiber itself. The fiber must be isolated from stresses which cause microbending. The properties of the fiber coating play a major role in providing this isolation, with coating geometry, modulus and thermal expansion coefficient being the most important factors.
Two types of coating materials are used commonly. Single coatings, employing a relatively high modulus, e.g. 10.sup.9 Pa, or an intermediate modulus, e.g. 10.sup.8 Pa, are used in applications requiring high fiber strengths or in cables which employ buffer tubes where fiber sensitivity to microbending is not a significant problem.
The problem of coating optical fibers becomes more complicated because of the functions the coating material must perform. Dual coated optical fibers increasingly are becoming used to obtain design flexibility and improved performance. Typically, a first or primary coating layer that comprises a relatively low modulus material, e.g. 10.sup.6 -10.sup.7 Pa, is applied to the optical fiber. Such a material reduces microbending losses associated with the cabling, installation or environmental changes during the service life of the optical fiber. An outer or secondary coating layer comprising a relatively high modulus material is applied over the primary layer. The outer coating layer is usually of a higher modulus material to provide abrasion resistance and low friction for the fiber and the primary coating layer. This structure isolates the fiber very well from external stresses which would tend to cause local bending. Such stresses may be imposed in two distinct ways. First, non-uniform lateral stresses, imposed by the cable structure surrounding the fiber, may cause bending with periodic components in the microbending regime. The dual coating serves to cushion the optical fiber via the primary layer and to distribute the imposed forces via the secondary layer, so as to isolate the optical fiber from bending moments. Secondly, axial compressive loading of the optical fiber occurs when the surrounding cable components contract relative to the fiber. Such contraction results from both the differential thermal contraction of cable materials relative to the glass fiber and from the viscoelastic recovery of residual orientation present in the cable materials. If the axial compressive load imposed on the optical fiber becomes large enough, the fiber will respond by bending or buckling. The low modulus primary coating is effective in promoting long bonding periods for the fiber which are outside the microbending range.
In one method of applying dual layers of coating materials to a moving optical fiber that is disclosed in U.S. Pat. No. 4,474,830 which issued on Oct. 2, 1984, in the name of C. R. Taylor, an optical fiber is passed through a coating applicator which includes first and second dies. The first die confines a first coating liquid which is maintained at a predetermined level in a reservoir above the first die over a portion of the fiber's length. A second coating liquid is applied onto the optical fiber through a clearance between the first and second dies. The clearance is sufficiently small so that substantially no circulation of the second coating liquid occurs in the vicinity of the point of application to the fiber. The second coating liquid which is applied includes a free surface in the immediate vicinity of the point of application to the fiber.
Notwithstanding the success of the above-identified C. R. Taylor arrangement, efforts have continued to apply a coating material to an optical fiber as the fiber is being moved at higher and higher manufacturing line speeds. Wanted is a system in which a coating material need not be maintained at a particular level and which does not require the system to hold a relatively large quantity of coating material that must be removed prior to cleaning and other maintenance. Further, it long has been a desire to improve the stringing up of an optical fiber on a draw tower and to reduce the amount of cooling which the optical fiber must experience after being drawn but prior to being coated.
Seemingly, the prior art does not include a coating arrangement which applies two coatings within a single applicator at relatively high line speeds, and which facilitates the string-up of the optical fiber with reduced cooling prior to coating. The sought-after methods and apparatus for coating should be easily and inexpensively implemented.